Set up GitHub Actions lints and checks

[Seeker14491]

This PR sets up GitHub Actions for linting and checking the code. For the backend, formatting is checked with rustfmt, and the clippy linter is ran on Windows, Mac, and Linux. For the frontend, we check for TypeScript errors with tsc and lint with ESLint.

Todo / Questions:

• [X] Document how to run the lints/formatter locally. We should probably provide a script in package.json.
• [X] Frontend code formatter? I'll implement this in a separate PR
• [X] There's a group of ESLint rules that are disabled by default for performance reasons. I tried enabling them, but there were 140+ errors reported so I left them off. Leave them off for now. I won't work on this as I'm not that comfortable with the frontend code.
• [X] Currently the Rust code uses 2-space indents, but 4-space indents is standard Rust style. Switch? Stay with 2-space

As part of this PR I fixed the warnings tsc and ESLint reported. I have minimal React knowledge though, so let me know if something's off.

[SpikeHD]

To answer your questions:

• Yes, we should provide a script in package.json
• If eslint doesn't handle this ok-enough already, we can implement prettier, but that may be better as a separate PR
• Lol fair, I think we can just leave that and I may come back to it separately
• 2-space has always been my preference and I would prefer to keep it that way but if anyone has any strong differing opinions I'm fine with changing it

All in all looks good to me as it currently stands